Leather tanning



United States Patent 3,254,938 LEATHER TANNING Pedro Villa Rodriguez, Juan Sanchez Azcona 430, Mexico City 12, Mexico, and Eusehio del Cueto de la Fuente, Grieta 165, Jardines del Pedregal, Mexico City 20, Mexico No Drawing. Filed Aug. 29, 1962, Ser. No. 220,154

5 Claims. (Cl. 8-94.27)

This invention relates to the processes for tanning of hides or skins to produce leather. It more particularly relates to a simplified process of chromium tanning wherein a new physical and chemical basis is used for tanning and the numerous steps required in the prior art processes for the treating of hides or skins prior to tanning are combined into a single tanning step and a superior tanned leather product is obtained in a shorter time.

In the prior art processes for producing leather, a large number of steps are required for preliminary treatment of the hides or skins prior to tanning, to dehair, plump open the structure of the hide and properly prepare the hide for tanning. For example, in The Chemistry and Technology of Leather, vol. 3, Reinhold Publishing Corp., New York, 1962 Edition, Chapters 37 and 38 discuss the numerous steps necessary to produce chrome tanned side upper leather. The' more important of these steps are briefly outlined below.

The hides and skins used for producing leather are, to a great extent, a by-product of the meat packing industry. They are generally preserved for shipping to tanneries either by treatment with salt rubbed on the flesh side, by soaking in a brine solution, by drying, or as fresh hides. In the salted condition, the hides are known as green salted hides and a typical analysis has shown that they have generally the following composition:

Percent Water 44.3 Salt 14.4 Protein 37.3 Fat 4.5

The skins are received in this condition at the tannery and the first object is to restore the skin to as near their original condition as possible. Thus, the first step at the tannery, after sorting and classifying of the skins, is to wash and soak the hides to restore water to the dried hides and to remove the salt or brine from the skins preserved in that manner.

The next treatment is for the removal of hair from the hide and to bring about the swelling of the fibers. The hide at this point consists of the outer hair, a thin outer layer known as the epidermis to which the hair is attached, the central layer known as the .corium and fats and greases either present within the corium or still attached to the flesh side of the hide. Treatment for removing of the hair from the hide is accomplished most generally in a lime bath. This is done by placing the hides in a water solution of lime for a period of time which may vary from one to four days or longer. Other agents may be added to the lime solution as is well known in the art to step up the activity of the lime bath.

In the case of dehairing of sheep skins, a preparation of lime and sodium sulphide is usually painted on the flesh side of the skins and allowed to remain there for a period of time after which the wool, which is a valuable by-product, may be pulled from the skin.

The limed hides, after the requisite period of time, are removed from the liming bath and the hair is stripped from the hide either by hand or by machine. Machine operations are also used to remove the epidermis and to remove any adherent particles on the flesh side of the hide.

The limed hides are then submitted to a de-liming treatment with, for example, ammonium salts, or lactic, acetic, formic, sulfuric or hydrochloric acids. The hides are then submitted to a bating treatment which prepares the skin for tanning. In the liming treatment, the skin swells and is brought to a high degree of alkalinity. The de-liming reduces the alkalinity of the hide and the hating, which is accomplished by the action of the enzymes, for example pancreatin, reduces the swelling of the hide and produces a smoother grain and prepares the fibers for tanning. The skins are next pickled in a bath with dilute sulfuric acid, which penetrates the structure of the skin and prepares it for the chromium salt action during the tanning. The pre-treated skins are then at an acid pH about 2.2 to 2.5.

The skins are then passed to a chromium bath consisting of an aqueous solution of chromium salts as is well known in the art. This bath is likewise at an acid pH of about 2.8 to 3.8. The skins are allowed to remain in the tanning bath for a sufficient length of time to convert the skin to leather. After the skins have been in the bath for this time, they are given an alkaline treatment with a weak sodium bicarbonate solution or a borax solution which brings about a fixation of the chrome salt within the fibers of the skin and produces a leather.

All of the above treatments have been used with various modifications by the leather industry since the inception of chrome tanning over years ago. Prior to this time, the pre-treatment of hides was likewise accomplished in a substantially similar manner for other tanning methods, such as vegetable tanning.

The above treatments are time-consuming and require considerable hand labor and chemicals, even though the treatments may take place in paddle-wheel vats or rotating drums on a semi-continuous basis. Moreover, close control is required in each step of the process. In addition, large quantities of water are required for all of the above steps and the water itself must be closely controlled as to temperature and natural mineral content. In addition to the amount of Water, the disposal of the water contaminated with the treatment chemicals often presents a serious problem.

The opportunities for error due to the numerous pretreatment steps and the close control required are great, making it diflicult to obtain a leather which is uniform from day to day.

Thus the manufacture of leather has remained relatively costly and the time-consuming work and the preparation for tanning has become the greatest cost item.

It is, therefore, an object of this invention to reduce the time required to obtain leather from hides and skins and to produce a superior leather.

Another object is to combine various pre-treatment operations into a single operation.

A further object of the invention is to provide a new physical and chemical basis for the tanning of skins and hides wherein tanning may be accomplished by means of a single pre-treatment step and a single tanning step.

Another object is to provide a method of producing many varieties of final leathers from one basic tannage.

A further object is to eliminate a large number of soaking, washing and other steps which require a large amount of water for the complete tanning process, thereby eifecting a greater economy in the process by reason of the amount of water required and the amount of sewage water to be treated before discharge.

Another object of the invention is to reduce the amount of chemicals required in the tanning process and to take advantage of the chemicals already present in the hides supplied to the tannery, for example, salt, bacteria, enzymes, etc., in the tanning operation.

Another object is to provide a tanning method whereby a hide or skin may be converted into leather with greater uniformity of the quality of the leather throughout all portions.

The above objects may be attained and the disadvantages of the prior art processes overcome by the use of our invention.

In principle, our process consists of subjecting the hides as received at the tannery to a controlled plumping, soaking and dehairing step in an alkaline treatment bath followed by mechanical dehairing and defleshing, after which the controlled plump skin, in an alkaline condition, is put directly into an acid chromium tanning bath either Without washing the alkali out of the dehaired skin or while the skin still remains in an alkaline condition. When the alkaline skin is put into the acid chromium tanning bath, an immediate reaction takes place between the alkaline fibers of the hide and the acid chrome tanning agent, whereby the chromium is uniformly and quickly combined throughout the entire hide or skin so as to produce a uniform tanning effect in a much shorter period than is possible with the tanning processes now in use. The process is illustrated in the following flow diagram:

HIDE

Green salted, raw, dried or any raw condition FIRST TREATING BATH-PREPARA'IION OF HIDE (DEPILATION, CONTROLLED SWELL- ING, ALKALIZING) [Aqueous alkaline solution-pH higher than 7, for example 8 to 14 pH] composifiion of Bath:

(1) ater (2) NaOl (added or from green salted hide) (3) Depilating agent: NaOH; N 84S; N aHS or etc.

Mechanical Dehairing and Defleshing SECOND TREATING BATHTANNIN G [Aqueous chromium tanning solution-pH less than 7, for example 3 to 4 pH] Composition of Bath: Standard basic chromium complex salt tanning solution (chromiinn sulfate, chloride or nitrate with hydroxyl groups in the complex) Whereas the present tanning processes taken several days or even weeks for the normal soaking, dehairing, liming, de-liming, bating, washing, pickling and tanning steps, requiring from 5 to 6 weeks from the hide house to the warehouse, our process can be carried out from the salted hide to the leather in a period of hours, varying from perhaps 4 to 22 hours, depending upon the size,

thickness and nature of the hide and the type of leather to be produced.

In contrast to the normal process of chrome tanning involving ten or more separate steps, our process consists essentially of only two steps; one, treatment of the hide or skin which may be a dried, salted or fresh hide in an alkaline bath with sodium sulfide, sodium hydroxide or other alkali buffered with sodium chloride in water and using, if necessary, other alkalies, buffers and salts to keep the first bath at a pH of approximately 8-14, after which the alkaline hide so treated is subjected to mechanical or manual dehairing, if necessary, and defleshing, and put directly into an acid chrome salt tanning bath. The object of the pretreatment is to put the hide into the proper physical and chemical condition for tanning. The hide or skin may or may not be washed between the alkaline treatment bath and the chrome tanning bath, and if it is washed, the washing is discontinued while the hide is still definitely on the alkaline side so that the reaction between the alkaline hide and the acid basic chrome salt tanning bath is an immediate and prompt reaction.

In the above pre-treatment step, the sodium sulfide swells and dehairs the hides or skins. The amount of water used is based on the weight of the skins and may vary anywhere from about 25 to 800%, preferably from 200 to 600% of the weight of the skins. The percent of the sodium sulfide may vary from 0.5 to 15%. Sodium chloride is added to the bath for buffering or controlling of the swelling action, since this prevents excessive swellmg. The quantity of sodium chloride should be at least about 0.25%. However, there is no upper limit for the sodium chloride. As it is well known, sodium sulfide breaks down into sodium hydroxide and sodium sulfhydrate in water. If it is desired to have a greater quantity of sodium hydroxide or sulphhydrate, the desired amount of either may be added as such. The sodium chloride required for buffering may be present in the hide if it is received in a green, salted condition, that is, preserved with salt or brine. In this case, there is no necessity for adding sodium chloride to the pre-treatment bath. The time for pro-treatment will vary from about 2 to 48 hours.

This pre-treatment may be accomplished either in vats, paddle-wheel vats, or drums in the conventional manner. We have found that the temperature of the pro-treatment bath is not critical and it is not necessary to control or regulate the temperature to produce good leather products. The treatment may be carried out at ambient temperatures and at the temperature of the water available. Moreover, no special control is required for the water.

the bath and dehairing and defleshing are mechanically accomplished.

The hide is then treated in a one bath conventional basic chrome salt bath while still in an alkaline condition. The length of time in the chrome tanning bath may vary from about 4 to 30 hours. It is not necessary to control the pH during tanning. No alkali treatment is required after the chrome'bath as in the conventional chrome tanning process described previously, since the skin is in an alkaline condition prior to the treatment, and this causes fixation of the chrome tanning salt in the leather. Thus, the complete tanning takes place in the two steps described above, with dehairing and defleshing between the two steps.

In the one-bath conventional chrome tanning process the actual tanning is generally accomplished by basic chromium sulfate. In the production of the tanning baths it is first necessary to produce the basic chromium salt from sodium dichromate. The basic chromium sulfate salt is a complex of chromium sulfate,'together with hydroxyl groups. The chrome complexes which are formed may be controlled by the method used for making the chrome liquor itself. The percentage of total chrome 'combined with hydroxy groups within a chrome complex is expressed in terms of basicity. The almost universally used method of expressing this basicity in a chrome complex is the Schorlemmer method which defines basicity as the percentage of the total primary valences of the chromium atoms present, which are occupied by hydroxyl groups. Therefore, if each chromium atom contains 1 hydroxyl group the complex is 33 /3 percent basic, since chromium has three valences. If it contains two hydroxyl groups it is 66% percent basic. Precipitated chromium hydroxide in the percent basic compound. In a normal range of chrome tanning essentially all the hydroxyl ions present in the solution are combined with the chromium and, therefore, the, basicity of the chrome complex is identical with the basicity of the solution. The

various basic chrome complexes may be illustrated by formula as follows:

Basicity Compound: (percent), Cr (SO 0 Cr(OH)(SO 33.3 Cr (OH) SO 66.7 Cr(OH) 100.0

In the preparation of the basic chrome tanning liquor the basicity may be controlled during the formation of the basic chrome complex salt. This salt can be produced by either sugar reduction or sulfur dioxide reduction of sodium dichromate in acid solution. In the first method, sugar and sulfuric acids are reacted with sodium dichromate; in the second method sulfur dioxide is added to an aqueous solution of the dichromate. In either case,

the basicity of the basic chrome complex can be con- The basicity of the tanning bath can be varied by the addition of alkali such as sodium hydroxide. This does not, however, give a built-in basicity such as that of the complexes and is not preferred.

What has been said about basicity above, may also be applied to acidity when considering the chrome complex and the chrome tanning solution. The acidity of the chrome complex is fixed by the number of acid forming ions present within the complex. However, the acidity of the chrome tanning solution can be varied by the addition of acid to the solution to increase the acid ions in the solution which are not a part of the chrome complex salt.

The preparation of basic chrome tanning bath for the conventional one-bath process which are used in this invention are discussed in greater detail in The Chemistry and Technology of Leather, vol. 2, cited above, in chapters 21 and 22.

It is clear that a reduction in the number of pre-treatment steps omitting such steps as soaking, liming, deliming, hating, pickling and combining all into one step accomplishes a great reduction in the quantity of water required. Generally, the amount of water used in our process is about to of the amount of water used in the prior processes. This is an economic advantage Where water must be purchased and pre-treated and it is also an advantage where water is not readily available. There is a further advantage in that less treatment water from the treating baths will have to be treated prior to going to sewage discharge. In addition, a great reduction in the number of chemicals is attained, due to the simplicity of the pre-treatment step.

Furthermore, the complexity of the tanning process is greatly reduced and this reduces the chance for error in each of the steps, since there is only one pretreatment step necessary to place the hide in condition for tanning. The amount of labor and time required for tanning leather is greatly reduced and a great economy in the production of leather results from this simplified process. In addition, the process may be applied to any type of leather, for example, cowhide, sheepskin, goatskin, horsehide, pigskin and the like, and to such specialty leathers as alligator, seal, ostrich, bufifalo, kangaroo, deerskin and the like.

The above twostep process is the basis of our invention and is the most simple and economic embodiment thereof.

It will, of course, be understood that other additives may be used in the process; for example, in the pretreatment process, surface active agents, sodium sulphates and sulfites, enzymes and the like may be used. With some types of leathers, the pre-treatment bath may consist of sodium hydroxide and sodium chloride alone. The essential :point is that the hide be put into an alkaline condition prior to the tanning step. After tanning, the leathers may be processed in any known, desired method, for example, they may be fat-liquored, stuffed, dried to a desired water content, staked, dyed and bufied. They may also receive a re-tanning treatment if this is desired for the final product, with vegetable or synthetic tannins. However, this re-tanning is not necessary to produce a satisfactory-leather. The leather may be split or shaved before or after tanning. The manner of operating the baths may be accomplished in a variety of Ways, as well known in the art, for example, the entire amount of chemicals may be added to the baths prior to placing the skins in the baths, or the skins may be first added; or the chemical agents may be added in increments as well known in the art.

After-tanning operations can be made to place the leather in the crust condition, from which it is easily Wetted back and finished when desired.

The following examples illustrate specific methods by which our new tanning processes can be practiced. It is to be understood, however, that they are only illustrative examples, that it is impossible to illustrate by example all of the manners and methods by which our fundamentally new tanning process can be practiced and that various modifications and variations of the examples given will readily occur to persons skilled in the art or arise naturally within the industry as different materials are to be tanned to produce leathers for different purposes, and that certain optimum conditions may be found. Moreover, While the process has been disclosed as applicable to chrome tanning, it may be used for tanning with other known mineral tanning agents. 'The examples are thus not meant in any way to limit the scope of the invention. 1

' EXAMPLE I For the production of shoe upper leather by the chrome tanning process, kg. of commercial green salted cattlehides were selected. The green salted hides had the. composition by analysis as follows:

Percent Water 44.3 Salt (NaCl) e 14.4 Protein 37.3 Fat -i 4.5

These hides were drummed at a normal speed of from 8-20 rpm. for 12 hours in a pre-treatment bath composition as follows:

Kgs. H O 394 Nags 6 EXAMPLE I I 100 kg. of cowhide from both heavy Mexican wet salted, and split brined United States hides were treated to prepare them for chrome tannin-g. The green salted hides were placed in a drum containing a solution as follows:

15! bath Kgs. H O 400 NaCl 5 Na s 2 N21250:; 1

The hides were then tumbled in the drum for about hours at a normal speed of about 8-20 r.p.m. The bath was at room temperature. After this time, the hides were taken out of the drum and dehaired and fleshed.

The hides thus treated were then, without washing or soaking or any further treatment, put into a drum containing a chrome salt tanning bath of the following composition:

2nd bath I Kgs. Cromosal SF (the chrome solution of Example I) 10 H O 3 The hides were drummed in the tanning bath for about 7 hours. After this time, the hides were removed from the drum.

The hides dried out into good, solid leather, having exceptionally tight break, smoothness, strength, and a very thick grain (without any retannage) that buffed extremely well. This is important for finishing hides having poor or damaged grain. A piece of the leather withstood a five minute boil test at 100 C. Thus, fully tanned leather was produced in a total time of 22 hours.

EXAMPLE III For producing chrome-tanned sole and upper leather, 100 kg. of green salted cattlehides were placed in a vat containing 600 kg. of a solution having 5' grams of Na S/Iiter and 5 grams of NaOH/liter, and left in the bath for hours.

The hides were then removed and weighed 120 kg. The hides Were treated in a bath of 120 kg. water and 6 kg. NaCl salt for 1 hour. Then solid salt was added to the bath until the desired degree of swelling of the hides was obtained.

The hides were then treated in 264 kg. of chrome liquor (200%) for 30 hours. A commercial chrome liquor was used having 24% Cr O 33% basicity, 35% Na SO (124 kgs. of the chromefsalt in 240 kgs. of H 0).

A full chrome tanned leather was produced. The pretreated hide above may he submitted to other treatments before tannin-g, such as simple water washing, treating with salt solutions o f different concentrations, with or without sodium hydroxide and/or surface active agents.

EXAMPLE IV 100 kg. of green salted cattlehides were treated to prepare them for chrome tanning with a pre-treat-ment hath having the calculated composition:

- Kgs.

Na S

The hides were placed in a drum, the drum was started and the solution was added in one strike. Drumming was continued for 12 hours.

.The hides were then removed from the drum and mechanically fleshed, dehaired and trimmed.

The prepared hides were then placed in a drum with a commercial chrome tanning hath .as in Example I of 30% of chrome liquor at 25 B. and drumming was continued for 12 hours. After this time, the hides were fully tanned into chrome leather suitable for shoe uppers, etc.

EXAMPLE v 1 00 kg. of green salted cattlehides were treated to prepare them for chrome tanning with a pretreatment bath having the calculated composition:

Kgs. H O 194 Na S 6 EXAMPLE VI 100 kg. of green salted cattlehides were treated to prepare them for chrome tanning with a pre-treatment bath having the calculated composition:

Kgs. H O 94 Nags 6 The hides were then placed in a drum, the drum started, and the solution was added as follows: first a part of the calculated water, drummed for a while, and then the rest of the. calculated water, which contained the calculated sulfide.

Drumming was continued for 12 hours.

The hides were then removed from the drum, fleshed, dehaired and trimmed.

The prepared hides were then placed in a drum with a commercial chrome tanning hath, as in Example I, of 40% of chrome liquor at 25 B., and drumming was continued for 12 hours. After this time, the hides were fully tanned into chrome leather suitable for shoe uppers, etc.

EXAMPLE VII 100 kg. of green salted cattlehides were treated to prepare them for chrome tanning with a pretreatment bath having the calculated composition:

The hides were placed in a drum, the drum was started and the solution was added as follows: first, a small part of the calculated water and the calculated sulfide (concentrated solution); hides were drummed for a while, and then the rest of the water added. Drumming was continued for 12 hours.

The hides were then removed from the drum, fleshed, dehaired and trimmed.

The prepared hides were then placed in a drum with a commercial chrome tanning bath, as in Example I, of 5 0% of chrome liquor at 25 B. and the drumming continued for 12 hours. After this time, the hides were fully tanned into chrome leather suitable for shoe uppers,

etc. a

9 EXAMPLE VIII 100 kg. of green salted cattlehides were treated to prepare them for chrome tanning with a pre-treatment bath having the calculated composition:

The hides were placed in a drum, the drum was started and the solution was added as follows: first part of the calculated water added; then, dissolved sulfide added, little by little. Drumming was continued for 12 hours.

The hides were then removed from the drum, fleshed, dehaired and trimmed.

The prepared hides were then placed in a drum with a commercial chrome tanning bath, as in Example I, of 25% of chrome liquor at 25 B. and drumming was continued for 12 hours. After this time, the hides were fully tanned into chrome leather suitable for shoe uppers, etc.

EXAMPLE IX 100 kg. of green salted cattlehides were treated to pre-.

pare them for chrome tanning with a pre-treatment bath having the calculated composition:

Kgs. H O 5-94 Na S 6 etc.

EXAMPLE X 100 kg. of green salted cattlehides were treated to pre pare them for chrome tanning with a pre-treatment bath having the calculated composition:

Kgs. H O 694 The calculated amount of sulfide was dissolved in a part of the calculated water so as to obtain a solution of sulfide in water, which was applied on the flesh side of the hide or skin, by the painting process; this solution may or may not be thickened with inert materials, such as clays, colloidal clays, kaolin or others. After about 5 to 6 hours, the hides were dehaired by hand or machine, washed or not, and put into the remainder of the calculated water (which may or may not contain sulfide, NaOH or other alkalis, and salt).

The hides were then removed from the drum, fleshed,

and trimmed.

The prepared hides were then placed in a drum with a commercial chrome tanning bath as in Example I, of of chrome liquor at B. and drumming was continued for 12 hours. After this time, the hides were fully tanned into chrome leather suitable for shoe uppers, etc.

EXAMPLE XI 100 kgs. of green salted cattlehides were treated to prepare them for chrome tanning with a pre-treatment bath having the calculated composition:

Kgs. H O u 794 Na S 6 Treatment was as follows: Part of-the calculated water was added, followed by drumming, so as to obtain an optimum proportion of water-hide-salt, a correct physical condition as to moisture. The hides were taken out and the paint system of Example X was applied. After dehairing, the rest of the water was added with or without salt and/or alkali. The hides were then removed from the drum, fleshed and trimmed.

The prepared hides were then placed in a drum with a commercial chrome tanning liquor as in Example I of 10% of chrome liquor at 25 B., and drumming was continued for 12 hours. After this time, the hides were fully tanned into chrome leather suitable for shoe uppers,

etc.

EXAMPLE XII kg. of green salted cattlehides were treated to prepare them for chrome tanning with a pre-treatment bath having the calculated composition:

Kgs. H O 400 Na s 4 100 kg. of green salted cattlehides were treated to prepare them for chrome tanning with a pre-treatment bath having the calculated composition:

Kgs. H O 294 NaOH 6 The hides were placed in a drum, the drum was started and the solution was added in one strike. Drumming was continued for 12 hours. The hides were then removed from the drum, fleshed, dehaired and trimmed.

The prepared hides were then placed in a drum with a commercial chrome tanning as in Example I of 30% of chrome liquor at 25 B. and drumming was continued for 12 hours. After this time, the hides were fully tanned into chrome leather suitable for shoe uppers, etc.

EXAMPLE XIV Dry sheep and goatskins with wool and hair, in the amount of 100 kgs. of dry skins were treated in 400 kgs. of a 3% salt solution for 48 hours. The skins were then fleshed and trimmed. Then the skins were painted on the flesh side with:

A 5% sodium sulfide solution (or up to 20%) thickened with kaolin to make a fluid paste. The solution was allowed to remain on the skins for 5 to 8 hours aftezl which they were dehaired, the dirt removed from the grain, washed and placed in:

200% of an 8% NaCl solution (on weight of skins) l to 2% of NaOH (on weight of skins) for 5 to 8 hours.

The skins were then taken out, wrung or centrifuged and placed directly into:

50% of a chrome liquor prepared by dissolving a basic chromium salt (commercial) of 24% Cr O 33% Sch. basicity, 30% Na SO of about 18 B. for 4 to 8 hours. Then the skins were taken out, wrung or centrifuged. A fully tanned, good quality leather was obtained.

A mixture of chrome salt and alum salt was also used,

in different proportions of each depending on the leather desired, and a partially neutralized solution of alum alone was used with good results.

1 1 EXAMPLE XV Dry sheep and goat skins, without wool or hair, in the amount of 100 kgs. of dry skins, were treated in a solution having the composition:

Kgs. 3% NaCl solution 400 Nazs 2 NaOH 2 for 30 hours. The skins were then unhaired, fleshed, washed and put back into the same solution. 50 kgs. of

salt (solid) were added little by little and stirred until the salt was dissolved. The hides were left 4 to 5 hours in this solution.

The hide was scudded and placed in the tanning bath,

of Example XIII.

A fullytanned leather was obtained from each type of skin.

EXAMPLE XVI 100 kgs. of calfskins was treated in a pre-treatment bath having a composition:

Kgs. H2O 294 Nags 6 for 12 hours.

The hides were then fleshed and dehaired. Next the hides were placed in a tanning bath having the composition:

K-gs. Alum 30 NaCl 15 H O 100 ,for 12 hours. Then the hides were taken out and wrung EXAMPLE XVII For the preparation of shoe upper leather, from green salted cattle hides, 18 to 25 kgs. per hide were taken from storage, sorted, trimmed, subjected to light fleshing, to betanned by the full chrome tanning process.

100 kgs. of the trimmed and lightly fleshed hides were treated in a drum with an alkaline solution made with:

400 H 0 6% of 60-62% Na S -based on the weight of the hides at an ambient temperature, normal r.p.m., with drumming for 12 hours.

The liquid phase was discharged and the skins were drummed dry in the drum for /2 hour. They were then taken out of the drum, fleshed, and deh-aired. Then the hides were put in another drum, the drum started and a commercial (33% Cr O 33.3% basicity Sch.) chrome salt: l0%=l2.5 kgs. dissolved in 37.5 kgs. of water in the conventional way. (Total liquor==50 kgs.) based on the 125 kgs. white or lime weight of the skins, was added to the drum.

Drumming was continued for 12 hours after which the hides were taken out. fully tanned chrome leather was obtained.

SPECIALTY LEATHERS The basic process for leathers known as specialty leathers is the same as for other leathers. However, since such leathers are for special purposes, they may be retanned with vegetable tanning agents to obtain the I Alligator skins in a green, salted condition were put directly into a salt-sodium hydroxide-water solution of about 3% salt, 3% NaOH, using about 400-600% of this solution based on the weight of the salted skins. The swelling of the skins was controlled with water or salt, to the desired degree. The skins were then Washed with water or a salt solution or a sodium hydroxide solution, and then put in a salt solution (about 12%) with or without 0.5-2% of sodium hydroxide for 24 hours The skins were removed from the bath, wrung or centrifuged, and placed into a chrome liquor as used in Example I. A fully tanned alligator leather was obtained. This leather was retanned with vegetable tannin to obtain an especially high luster.

The same process can be applied to pigskins, snakeskins, lizards, other reptiles, fishes, birds (ostrich, etc.), and other skins.

EXAMPLE XIX sodium hydroxide solution, and then put in a salt solution (about 12%) with or without 0.5-2% of sodium hydroxide for 20 hours. The skins were removed from the bath, wrung or centrifuged, and placed into a chrome liquor as used in Example I. A fully tanned pigskin leather was obtained.

While we have set forth certain specific embodiments and preferred modes of operation of our invention, it will be understood that these are for the purpose of illustration and that the invention is not limited thereto, and may be changed or modified without departing from the spirit of the disclosure or the scope of the appended claims.

We claim:

1. The method of producing chrome tanned leather which comprises the following steps in stated order: (1) subjecting the raw hide to treatment in an aqueous alkaline treating bath at a pH of from 8 to 14, said bath containing an alkali consisting of compounds selected from the group consisting of sodiumhydroxide, sodium sulfide, sodium sul fohydrate and mixtures thereof and a buffering agent consisting essentially of at least 0.25% of sodium chloride, based on the weight of the hide, until the hide is in an alkaline condition and has been plumped to the desired degree and the hair and flesh prepared for removal; (2) removing the hair and flesh from the hide; (3) subjecting said dehaired and defleshed hide in an alkaline condition to treatment in an aqueous tanning bath having an acid pH below 7 containing essentially as a sole tanning agent a one-bath conventional basified chrome tanning agent for a period of fromabout 4 to 30 hours, and thereafter removing chrome tanned leather from said tanning bath.

2. The process of claim 1 wherein the alkali is sodium sulfide.

3. The process of claim 1 wherein the alkali is sodium hydroxide.

4. The method of producing chrome tanned leather which comprises subjecting the raw hide to treatment in an aqueous alkaline treating bath at a pH of from 8 to 14, said bath containing from about 0.5 to 15% sodium sulfide and at least 0.25% of sodium chloride, based on the weight of the hide, until the hide has been plumped and the hair and fiesh prepared for removal, the amount of said bath being from about 25 to 800% by Weight based on the hide treated, removing the hair and flesh from the hide, subjecting the said dehaired and defleshed hide in an alkaline condition to treatment in an aqueous tanning bath having an acid pH below 7 containing essentially as a sole tanning agent a one-bath conventional basified chrome tanning agent for a period of from about 4 to 30 hours, and thereafter removing said chrome tanned leather from said tanning bath.

5. The method of producing chrome tanned leather which comprises subjecting the raw hide to treatment in an aqueous alkaline treating bath at a pH of from 8 to 14, said bath containing from about 0.5 to 15% sodium sulfide and at least 0.25% of sodium chloride, based on the weight of the hide, for a period of about 2 to 48 hours, until the hide has been plumped, and the hair and flesh prepared for removal, the amount of said bath being from about 25 to 800% by weight based on the hide treated, removing the hair and flesh from the hide, subjecting said dehaired and defleshed hide in an alkaline condition to treatment in an aqueous tanning bath having an acid pH below 7 containing essentially as a sole tanning agent a one-bath conventional basified chrome tanning agent for a period of from about 4 to 30 hours, and thereafer removing said chrome tanned leather from said tanning bath.

References Cited by the Examiner UNITED STATES PATENTS 1,982,586 11/ 1934 Vogel 894.27 2,707,667 5/1955 Severn 8-94.26 3,174,817 3/1965 Spahrkas 8-94.27 X

FOREIGN PATENTS 121,525 11/ 1959 Russia.

8/ 1962 Russia.

OTHER REFERENCES McLaughlin, The Chemistry of Leather Manufacture, pp. 198-201, pub. by Reinhold Publ. O0rp., 1945.

Wilson, The Chemistry of Leather Manufacture 2nd ed., vol. 1, p. 385, pub. 1928 by The Chem. Catalog Co. Inc., New York.

Lamb, The Manufacture of Chrome Leather, pp. 95 and 117, pub. 1923 by the Anglo-American Technical Co. Ltd., London, England.

Wilson, Modern Practice in Leather Manufacture, p. 201, pub. by Reinhold Publ. Corp., New York, N.Y., 1941.

J. TRAVIS BROWN, Acting Primary Examiner.

ABRAHAM H. WINKELSTEIN, NORMAN G.

TORCHIN, Examiners.

' D. LEVY, Assistant Examiner. 

1. THE METHOD OF PRODUCING CHROME TANNED LEATHER WHICH COMPRISES THE FOLLOWING STEPS IN STATED ORDER: (1) SUBJECTING THE RAW HIDE TO TREATMENT IN AN AQUEOUS ALKALINE TREATING BATH AT A PH OF FROM 8 TO 14, SAID BATH CONTAINING AN ALKALI CONSISTING OF COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF SODIUM HYDROXIDE, SODIUM SULFIDE, SODIUM SULFOHYDRATE AND MIXTURES THEREOF AND A BUFFERING AGENT CONSISTING ESSENTIALLY OF AT LEAST 0.25% OF SODIUM CHLORIDE, BASED ON THE WEIGHT OF THE HIDE, UNTIL THE HIDE IS IN AN ALKALINE CONDITION AND HAS BEEN PLUMPED TO THE DESIRED DEGREE AND THE HAIR AND FLESH PREPARED FOR REMOVAL; (2) REMOVING THE HAIR AND FLESH FROM THE HIDE; (3) SUBJECTING SAID DEHAIRED AND DEFLESHED HIDE IN AN ALKALINE CONDITION TO TREATMENT IN AN AQUEOUS TANNING BATH HAVING AN ACID PH BELOW 7 CONTAINING ESSENTIALLY AS A SOLE TANNING AGENT A ONE-BAT CONVENTIONAL BASIFIED CHROME TANNING AGENT FOR A PERIOD OF FROM ABOUT 4 TO 30 HOURS, AND THEREAFTER REMOVING CHROME TANNED LEATHER FROM SAID TANNING BATH. 